1. Field
The invention relates generally to holographic data storage media and systems, and more particularly to methods and systems for recording and/or reading holographic storage media having a reflective layer for readout (or playback).
2. Description of Related Art
Holographic data storage systems store information or data based on the concept of a signal beam interfering with a reference beam at a holographic storage medium. The interference of the signal beam and the reference beam creates a holographic representation, i.e., a hologram, of data elements as a pattern of varying refractive index and/or absorption imprinted in a volume of a storage or recording medium such as a photopolymer or photorefractive crystal. Combining a data-encoded signal beam, referred to as an object beam, with a reference beam can create the interference pattern at the storage medium. A spatial light modulator (SLM) or lithographic data mask, for example, may create the data-encoded signal beam. The interference pattern induces material alterations in the storage medium that generate the hologram.
The formation of the hologram in the storage medium is generally a function of the relative amplitudes and polarization states of, and phase differences between, the signal beam and the reference beam. The hologram is also dependent on the wavelengths and angles at which the signal beam and the reference beam are projected into the storage medium. After a hologram is created in the storage medium, projecting the reference beam into the storage medium interacts and reconstructs the original data-encoded signal beam. The reconstructed signal beam may be detected by using a detector, such as a CMOS photo-detector array or the like. The recovered data may then be decoded by the photo-detector array into the original encoded data.
A basic holographic system is illustrated in FIG. 1. The holographic storage system includes a light source 110, for example, a laser for providing a coherent beam of light. A beam splitter 114 is positioned to split the laser beam into an object beam and a reference beam. The object beam is directed to an SLM or data mask 116 where it is encoded with information as a two-dimensional image and directed to the recording storage medium 124 by mirror 118 and lens 120 where it interferes with the reference beam directed via mirror 130. A complex interference pattern is recorded in the storage medium 124 where the object beam and reference beam interact. After a first image or layer is recorded, the system may be modified to enable additional images to be recorded in storage medium 124. For example, by modifying the angle and/or wavelength of the reference beam, successive images may be recorded in the storage medium 124.
A particular image may be retrieved from recording medium 124 with a reference beam similar to the original reference beam used to store the image. The light is diffracted by storage medium 124 according to the stored hologram and the two-dimensional image that was stored in recording medium 124 is directed by lens 126 to photo-detector array 128.
Two basic holographic system geometries include transmission and reflection geometry. In transmission geometry, shown in FIGS. 1 and 2, the diffracted light from the hologram exits the media from the opposite side from the incident reference beam. The light source, e.g., a laser source, and the camera for detection are therefore disposed on opposite sides of the media. For recording at different spatial locations on the media, such systems are typically limited to moving the media because of the complexity of synchronously moving the laser source and camera on both sides of the media, if the media were kept stationary.
In reflection geometry, shown in FIG. 3, the diffracted light from the hologram exits the media from the same side as the incident reference beam. Because the laser source and camera are on the same side of the media in this case, this geometry is more flexible for either moving the media or moving the laser source and camera (e.g., together on a shared head or stage) in order to access different locations on the media.
Holograms recorded in reflection geometry, however, are generally more sensitive than transmission geometry holograms to effects such as shrinkage or anisotropic thermal expansion of the media. Such media distortion leads to detuning of the beam angles needed to properly read out the holograms, and are about an order of magnitude larger for reflection geometry holograms than for transmission geometry. For high bandwidth object beams, which span a wide angular range, different parts of the image can have significant variation in detuning angles, so that it may not be possible to fully recover the entire data page. The magnitude of the variation is smaller for transmission geometry, and furthermore, it can be compensated almost fully with a proper combination of readout beam angle and wavelength adjustments. For reflection geometry, even with an optimal adjustment of angle and wavelength, the entire data page may not be recoverable.
One architecture variation which has been proposed previously is to use media with a reflective layer on one side, such as shown in FIG. 4. Such an example is described in Saito, K. and Horimai, H. (1998) “Holographic 3-D Disk using In-Line Face-to-Face Recording”, Optical Media Laboratory, Sony Corporation, pp. 162-164, the entire content of which is hereby incorporated by reference. This has a similar benefit of reflection geometry, i.e., having all components on one side of the media; however, the reflective layer is present for both recording and readout. This has the consequence that during recording, the hologram area has a mixture of incident and reflected beams for both the reference and object beams. As a result, both transmission and reflection hologram components are recorded in the same volume. As described earlier, under shrinkage or thermal expansion, the reflection and transmission hologram components will behave differently with different degrees of detuning. This can lead to interference and distortion between the transmission and reflection components of the reconstructed hologram.
Another prior art variation includes the use of a reflective layer together with a polarization shifting layer. Such an example is described, for example, in U.S. Pat. No. 6,721,076, to King, B., Anderson, K., and Curtis, K., entitled “SYSTEM AND METHOD FOR REFLECTIVE HOLOGRAPHIC STORAGE WITH ASSOCIATED MULTIPLEXING TECHNIQUES,” the entire content of which is hereby incorporated by reference. In this case, both reflected beam polarizations are rotated upon reflection, so that transmission hologram components are recorded by both the incident and reflected beam pairs, but no reflection hologram components are recorded. This avoids the possible negative interaction between transmission and reflection components of the reconstructed hologram, but generally benefits from the incorporation of a potentially costly polarization shifting layer in the media fabrication. Also, even though no holograms are recorded between beam components with crossed polarizations, the presence of light from the reflected beams in the same volume where the hologram between the incident beams is being recorded, and vice versa, may reduce the modulation depth of the holograms, resulting in wasted dynamic range of the media.